Method for detecting transverse vibrations in an ultrasonic hand piece

ABSTRACT

A method for detecting transverse mode vibrations in an ultrasonic hand piece/blade is achieved by monitoring the power delivered to the hand piece/blade to determine whether it increases as expected when power levels applied to the hand piece/blade are changed. While the blade is being held in midair, the power delivered to the hand piece/blade and/or the impedance of the hand piece/blade is measured at a first power level. Using the value obtained at the first power level, the expected power at a second power level is calculated and used to set a pass/fail threshold level for an actual measured power. Alternatively, the threshold is set for the impedance is set. Next, the actual power delivered to the hand piece/blade and/or the impedance of the hand piece/blade is measured at a level  5  power setting. A determination is made whether the hand piece/blade exhibits transverse mode behavior based on whether the actual measured power exceeds the established pass/fail threshold level. If this is the case, operation of the generator is inhibited, a “Transverse Mode Vibrations Present in Hand Piece/Blade” error code is stored in the generator, and a “Bad Hand Piece” message is displayed on a liquid crystal display on the generator. In addition, a method for detecting transverse mode vibration in an ultrasonic hand piece/blade is achieved by monitoring power delivered to the hand piece/blade to determine if the an extraordinary power increase occurs as the drive frequency is shifted downward and/or upward from a primary intended resonance operating frequency.

RELATED APPLICATIONS

The present invention relates to, and claims priority of, U.S.Provisional Patent Application Serial No. 60/242,251 filed on Oct. 20,2000, having the same title as the present invention, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to ultrasonic surgical systems and, moreparticularly, to a method for detecting transverse mode vibrations in anultrasonic hand piece/blade.

2. Description of the Related Art

It is known that electric scalpels and lasers can be used as a surgicalinstrument to perform the dual function of simultaneously effecting theincision and hemostatis of soft tissue by cauterizing tissues and bloodvessels. However, such instruments employ very high temperatures toachieve coagulation, causing vaporization and fumes as well assplattering. Additionally, the use of such instruments often results inrelatively wide zones of thermal tissue damage.

Cutting and cauterizing of tissue by means of surgical blades vibratedat high speeds by ultrasonic drive mechanisms is also well known. One ofthe problems associated with such ultrasonic cutting instruments isuncontrolled or undamped vibrations and the heat, as well as materialfatigue resulting therefrom. In an operating room environment attemptshave been made to control this heating problem by the inclusion ofcooling systems with heat exchangers to cool the blade. In one knownsystem, for example, the ultrasonic cutting and tissue fragmentationsystem requires a cooling system augmented with a water circulatingjacket and means for irrigation and aspiration of the cutting site.Another known system requires the delivery of cryogenic fluids to thecutting blade.

It is known to limit the current delivered to the transducer as a meansfor limiting the heat generated therein. However, this could result ininsufficient power to the blade at a time when it is needed for the mosteffective treatment of the patient. U.S. Pat. No. 5,026,387 to Thomas,which is assigned to the assignee of the present application and isincorporated herein by reference, discloses a system for controlling theheat in an ultrasonic surgical cutting and hemostasis system without theuse of a coolant, by controlling the drive energy supplied to the blade.In the system according to this patent an ultrasonic generator isprovided which produces an electrical signal of a particular voltage,current and frequency, e.g. 55,500 cycles per second. The generator isconnected by a cable to a hand piece which contains piezoceramicelements forming an ultrasonic transducer. In response to a switch onthe hand piece or a foot switch connected to the generator by anothercable, the generator signal is applied to the transducer, which causes alongitudinal vibration of its elements. A structure connects thetransducer to a surgical blade, which is thus vibrated at ultrasonicfrequencies when the generator signal is applied to the transducer. Thestructure is designed to resonate at the selected frequency, thusamplifying the motion initiated by the transducer.

The signal provided to the transducer is controlled so as to providepower on demand to the transducer in response to the continuous orperiodic sensing of the loading condition (tissue contact or withdrawal)of the blade. As a result, the device goes from a low power, idle stateto a selectable high power, cutting state automatically depending onwhether the scalpel is or is not in contact with tissue. A third, highpower coagulation mode is manually selectable with automatic return toan idle power level when the blade is not in contact with tissue. Sincethe ultrasonic power is not continuously supplied to the blade, itgenerates less ambient heat, but imparts sufficient energy to the tissuefor incisions and cauterization when necessary.

The control system in the Thomas patent is of the analog type. A phaselock loop (that includes a voltage controlled oscillator, a frequencydivider, a power switch, a matching network and a phase detector),stabilizes the frequency applied to the hand piece. A microprocessorcontrols the amount of power by sampling the frequency, current andvoltage applied to the hand piece, because these parameters change withload on the blade.

The power versus load curve in a generator in a typical ultrasonicsurgical system, such as that described in the Thomas patent, has twosegments. The first segment has a positive slope of increasing power asthe load increases, which indicates constant current delivery. Thesecond segment has a negative slope of decreasing power as the loadincreases, which indicates a constant or saturated output voltage. Theregulated current for the first segment is fixed by the design of theelectronic components and the second segment voltage is limited by themaximum output voltage of the design. This arrangement is inflexiblesince the power versus load characteristics of the output of such asystem can not be optimized to various types of hand piece transducersand ultrasonic blades. The performance of traditional analog ultrasonicpower systems for surgical instruments is affected by the componenttolerances and their variability in the generator electronics due tochanges in operating temperature. In particular, temperature changes cancause wide variations in key system parameters such as frequency lockrange, drive signal level, and other system performance measures.

In order to operate an ultrasonic surgical system in an efficientmanner, during startup the frequency of the signal supplied to the handpiece transducer is swept over a range to locate the resonancefrequency. Once it is found, the generator phase lock loop locks on tothe resonance frequency, continues to monitor the transducer current tovoltage phase angle, and maintains the transducer resonating by drivingit at the resonance frequency. A key function of such systems is tomaintain the transducer resonating across load and temperature changesthat vary the resonance frequency. However, these traditional ultrasonicdrive systems have little to no flexibility with regards to adaptivefrequency control. Such flexibility is key to the system's ability todiscriminate undesired resonances. In particular, these systems can onlysearch for resonance in one direction, i.e., with increasing ordecreasing frequencies and their search pattern is fixed. The systemcannot: (i) hop over other resonance modes or make any heuristicdecisions, such as what resonance to skip or lock onto, and (ii) ensuredelivery of power only when appropriate frequency lock is achieved.

The prior art ultrasonic generator systems also have little flexibilitywith regard to amplitude control, which would allow the system to employadaptive control algorithms and decision making. For example, thesefixed systems lack the ability to make heuristic decisions with regardsto the output drive, e.g., current or frequency, based on the load onthe blade and/or the current to voltage phase angle. It also limits thesystem's ability to set optimal transducer drive signal levels forconsistent efficient performance, which would increase the useful lifeof the transducer and ensure safe operating conditions for the blade.Further, the lack of control over amplitude and frequency controlreduces the system's ability to perform diagnostic tests on thetransducer/blade system and to support troubleshooting in general.

Some limited diagnostic tests performed in the past involve sending asignal to the transducer to cause the blade to move and the system to bebrought into resonance or some other vibration mode. The response of theblade is then determined by measuring the electrical signal supplied tothe transducer when the system is in one of these modes. The ultrasonicsystem described in U.S. Application Ser. No. 09/693,621, filed on Oct.20, 2000, which is incorporated herein by reference, possesses theability to sweep the output drive frequency, monitor the frequencyresponse of the ultrasonic transducer and blade, extract parameters fromthis response, and use these parameters for system diagnostics. Thisfrequency sweep and response measurement mode is achieved via a digitalcode such that the output drive frequency can be stepped with highresolution, accuracy, and repeatability not existent in prior artultrasonic systems.

Another problem associated with the prior art ultrasonic systems isunwanted vibrations in the hand piece/blade. Ultrasonic blades alsovibrate along an axis which is perpendicular to the longitudinal axis ofvibration of the hand piece/blade. Such vibrations are called transversemode vibrations. If the longitudinal vibration is considered to be inthe Z direction in an X, Y, Z coordinate system, vibrations along aY-axis of the blade are called transverse “flap mode” vibrations andvibrations along an X-axis of the blade are called transverse “hookmode” vibrations. Blades typically have a sheath surrounding their bladepart.

Transverse mode vibrations generate heat, which leads to high bladeand/or blade sheath temperatures. This can damage tissue surrounding anindented narrow cut or coagulation zone, thus adversely affectingpatient healing and recovery time. In addition, transverse modevibrations can cause blade tip failures. The vibrations may also beindicative of defects in the hand piece, such as damaged transducerdisks. While excess transverse mode vibrations are sometimes annoyinglyaudible, often a user will ignore them for as long as possible. It istherefore advantageous to detect transverse mode vibrations to preventundesired effects, such as tissue damage which can occur from an overheated blade.

SUMMARY OF THE INVENTION

The invention is a method for detecting transverse mode vibrations in anultrasonic transducer/blade. With this method, the power delivered tothe hand piece/blade is monitored at multiple power levels to determinewhether it changes as expected when the power levels applied to the handpiece/blade are changed. (Power levels are associated with specificcurrents at which the generator drives the hand piece/blade, regardlessof load changes on the blade.)

Transverse mode vibrations are excited by (non-linear) interactions oflongitudinal vibrations with the hand piece/blade. These vibrations bendthe blade, thereby causing the generation of heat, which drains energyfrom the desired longitudinal vibrations. This energy drain manifestsitself as an increase in hand piece/blade impedance, therebynecessitating an increase in the power delivered to the hand piece/bladeto maintain the required current through the transducer of the handpiece.

The non-linear mechanical coupling of energy from longitudinal totransverse vibrations is appreciable only above certainenergy/displacement thresholds. Therefore, if the impedance seen at alow “reference” power level is less than the impedance of the same handpiece/blade at a higher power level, then transverse vibrations are morethan likely present. In embodiments, a power measurement at a low powerlevel under test is used to calculate an expected power at a high powerlevel.

A reference power consumption measurement at a low power level settingis performed. This measurement is used to establish pass/fail powerconsumption levels for a high power level setting. The reference powerlevel measurement is essential, because the transducer/blade impedanceseen by the generator depends on the blade used.

In accordance with the invention, while the blade is being held inmidair, the power delivered to the hand piece/blade is measured at a lowpower level setting, where the drive current is low and does not triggertransverse vibrations. Using the value obtained at the low drive currentlevel setting, the expected power at a second high power is calculatedand used to set a pass/fail threshold for a second measurement at thehigh power level setting. Next, the actual power delivered to the handpiece/blade is measured at the high power level setting, and adetermination is made whether the hand piece/blade exhibits transversemode vibrations based on whether the actual measured power at the highpower level setting exceeded the established pass/fail threshold level.If this is the case, operation of the generator is inhibited, a“Transverse Mode Vibrations Present in Hand Piece/Blade” error code isstored in the generator, and a “Bad Hand Piece” message is displayed onthe LCD of the console.

In accordance with an embodiment, while the blade is being held inmidair, the drive current level is swept from a minimum drive current toa maximum drive current. During the current sweep, the transducervoltage and current drive signals are monitored and stored innon-volatile memory located in the generator. Using the stored voltageand current data, the power delivered to the transducer is calculated,and the Power-Delivered vs. Drive Current and Hand Piece/Blade Impedancevs. Drive Current response curves are generated. Using the generatedresponse curves, an extrapolation is performed to determine whether theHand Piece/Blade exhibits transverse mode vibrations. If this is thecase, operation of the generator is inhibited, a “Transverse ModeVibrations Present in Hand Piece/Blade” error code is stored in thegenerator, and a “Bad Hand Piece” message is displayed on the LCD of theconsole.

In an alternative embodiment, a Multiple Level Drive Power vs. PowerDelivered relationship and/or a Multiple Level Drive Power vs. Impedancerelationship is used to detect or predict potential transverse modeproblems, along with an “over-drive” of the hand piece at one or severalpower drive levels beyond the normal range of power levels used. These“over-drive” power levels are particularly effective at rapidlyidentifying problematic or potentially problematic transverse modeconditions.

In another embodiment, the power delivered to the hand piece is measuredat multiple frequencies while a high power drive signal is applied tothe hand piece. Alternatively, an “over-drive” is used. Here, threefrequencies, i.e., a first frequency, a second frequency and a thirdfrequency, are measured in close proximity to each other. The firstfrequency is the primary resonance frequency, otherwise referred to asthe main or intended longitudinal resonance frequency of the handpiece/blade. The second frequency is slightly below the first frequency.The third frequency is slightly above the first frequency. The expectedimpedance or power increases somewhat at both the second and thirdfrequencies. If the impedance or power is substantially higher or higherthan expected, this condition indicates the presence of a transverseresonance, which may create undesired heat and/or reduce ultrasonicenergy delivered into tissue. In this case, an alert or alarm isgenerated by the generator console. If necessary, a handicap limitedfunctionality or complete disabling of the hand piece drive isperformed.

Instead of monitoring impedance or power delivered to the hand piece,other variables can be monitored for comparison, such as the phase, thecurrent, the voltage, a power factor, or the like. Alternately, ratherthan comparing the primary frequency measurements to both a slightlyhigher and a slightly lower frequency measurement, the comparison isperformed at only the “second frequency” or at only the “thirdfrequency.” As a result, the monitoring process is accelerated in caseswhere monitoring additional frequencies is not necessary or needed.

In another embodiment of the invention, while the blade is being held inmidair or on tissue, the power delivered to the hand piece/blade ismeasured at first and second frequencies. Using the values obtained atthe first and second frequencies, the expected power at a thirdfrequency, a fourth frequency and a fifth frequency are calculated andused to set a pass/fail threshold level for an actual measured thirdpower through an actual measured fifth power, respectively. Next, theactual power delivered to the hand piece/blade is measured at the third,fourth, and fifth frequencies. A determination is made whether the handpiece/blade exhibits transverse mode vibration based on whether any ofthe actual measured powers exceed the established pass/fail thresholdlevels. If this is the case, operation of the generator is inhibited,and an alarm/alert message and/or audible alarm/alert is generated.Rather than monitoring the power delivered to the hand piece, inalternative embodiments other variables are monitored for comparisonpurposes, such as the phase, the impedance, the current, the voltage,the power factor, the phase margin, or the like.

In a further embodiment of the invention, the occurrence of whether atransverse frequency is located near the intended drive resonance isdetermined, and difficulties associated with the detection of transversemodes stimulated by the primary/main resonance drive frequency at highpower are resolved.

The method provides an indication of whether a hand piece, which failedthe power level tests, will exhibit transverse vibrational modes if itis used. In addition, the method eliminates the need to know thespecific type of blade being used with the hand piece during diagnostictesting.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of the preferredembodiments of the invention given below with reference to theaccompanying drawings in which:

FIG. 1 is an illustration of a console for an ultrasonic surgicalcutting and hemostasis system, as well as a hand piece and foot switchin which the method of the present invention is implemented;

FIG. 2 is a schematic view of a cross section through the ultrasonicscalpel hand piece of the system of FIG. 1;

FIGS. 3(a) and 3(b) are block diagrams illustrating an ultrasonicgenerator for implementing the method of the present invention;

FIG. 4 is a table showing the maximum current setting associated withvarious power levels at which the hand piece is driven;

FIGS. 5(a) and 5(b) are flowcharts illustrating an embodiment of themethod of the invention;

FIGS. 6(a) and 6(b) are flowcharts illustrating an embodiment of theinvention;

FIG. 7 is a flowchart illustrating an alternative embodiment of theinvention;

FIGS. 8(a) and 8(b) are flowcharts illustrating a preferred embodimentof the invention;

FIG. 9 is an illustration of a plot of transducer/blade powerconsumption at various generator power level settings when transversevibrations are not present in the hand piece/blade;

FIG. 10 is an illustration of a plot of transducer/blade powerconsumption at various generator power level settings when transversevibrations are present in the hand piece/blade;

FIG. 11 is an illustration of a plot of transducer/blade impedance atvarious generator power level settings when transverse vibrations arenot present in the hand piece/blade; and

FIG. 12 is an illustration of a plot of transducer/blade impedance atvarious generator power level settings when transverse vibrations arepresent in the hand piece/blade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an illustration of a system for implementing the method inaccordance with the invention. By means of a first set of wires in cable20, electrical energy, i.e., drive current, is sent from the console 10to a hand piece 30 where it imparts ultrasonic longitudinal movement toa surgical device, such as a sharp scalpel blade 32. This blade can beused for simultaneous dissection and cauterization of tissue. The supplyof ultrasonic current to the hand piece 30 may be under the control of aswitch 34 located on the hand piece, which is connected to the generatorin console 10 via wires in cable 20. The generator may also becontrolled by a foot switch 40, which is connected to the console 10 byanother cable 50. Thus, in use a surgeon may apply an ultrasonicelectrical signal to the hand piece, causing the blade to vibratelongitudinally at an ultrasonic frequency, by operating the switch 34 onthe hand piece with his finger, or by operating the foot switch 40 withhis foot.

The generator console 10 includes a liquid crystal display device 12,which can be used for indicating the selected cutting power level invarious means such, as percentage of maximum cutting power or numericalpower levels associated with cutting power. The liquid crystal displaydevice 12 can also be utilized to display other parameters of thesystem. Power switch 11 is used to turn on the unit. While it is warmingup, the “standby” light 13 is illuminated. When it is ready foroperation, the “ready” indicator 14 is illuminated and the standby lightgoes out. If the unit is to supply maximum power, the MAX button 15 isdepressed. If a lesser power is desired, the MIN button 17 is activated.The level of power when MIN is active is set by button 16.

When power is applied to the ultrasonic hand piece by operation ofeither switch 34 or 40, the assembly will cause the surgical scalpel orblade to vibrate longitudinally at approximately 55.5 kHz, and theamount of longitudinal movement will vary proportionately with theamount of driving power (current) applied, as adjustably selected by theuser. When relatively high cutting power is applied, the blade isdesigned to move longitudinally in the range of about 40 to 100 micronsat the ultrasonic vibrational rate. Such ultrasonic vibration of theblade will generate heat as the blade contacts tissue, i.e., theacceleration of the blade through the tissue converts the mechanicalenergy of the moving blade to thermal energy in a very narrow andlocalized area. This localized heat creates a narrow zone ofcoagulation, which will reduce or eliminate bleeding in small vessels,such as those less than one millimeter in diameter. The cuttingefficiency of the blade, as well as the degree of hemostasis, will varywith the level of driving power applied, the cutting rate of thesurgeon, the nature of the tissue type and the vascularity of thetissue.

As illustrated in more detail in FIG. 2, the ultrasonic hand piece 30houses a piezoelectric transducer 36 for converting electrical energy tomechanical energy that results in longitudinal vibrational motion of theends of the transducer. The transducer 36 is in the form of a stack ofceramic piezoelectric elements with a motion null point located at somepoint along the stack. The transducer stack is mounted between twocylinders 31 and 33. In addition a cylinder 35 is attached to cylinder33, which in turn is mounted to the housing at another motion null point37. A horn 38 is also attached to the null point on one side and to acoupler 39 on the other side. Blade 32 is fixed to the coupler 39. As aresult, the blade 32 will vibrate in the longitudinal direction at anultrasonic frequency rate with the transducer 36. The ends of thetransducer achieve maximum motion with a portion of the stackconstituting a motionless node, when the transducer is driven with amaximum current at the transducers' resonant frequency. However, thecurrent providing the maximum motion will vary with each hand piece andis a valve stored in the non-volatile memory of the hand piece so thesystem can use it.

The parts of the hand piece are designed such that the combination willoscillate at the same resonant frequency. In particular, the elementsare tuned such that the resulting length of each such element isone-half wavelength. Longitudinal back and forth motion is amplified asthe diameter closer to the blade 32 of the acoustical mounting horn 38decreases. Thus, the horn 38 as well as the blade/coupler are shaped anddimensioned so as to amplify blade motion and provide harmonic vibrationin resonance with the rest of the acoustic system, which produces themaximum back and forth motion of the end of the acoustical mounting horn38 close to the blade 32. A motion at the transducer stack is amplifiedby the horn 38 into a movement of about 20 to 25 microns. A motion atthe coupler 39 is amplified by the blade 32 into a blade movement ofabout 40 to 100 microns.

The system which creates the ultrasonic electrical signal for drivingthe transducer in the hand piece is illustrated in FIGS. 3(a) and 3(b).This drive system is flexible and can create a drive signal at a desiredfrequency and power level setting. A DSP 60 or microprocessor in thesystem is used for monitoring the appropriate power parameters andvibratory frequency as well as causing the appropriate power level to beprovided in either the cutting or coagulation operating modes. The DSP60 or microprocessor also stores computer programs which are used toperform diagnostic tests on components of the system, such as the handpiece/blade.

For example, under the control of a program stored in the DSP ormicroprocessor 60, such as a phase correction algorithm, the frequencyduring startup can be set to a particular value within a range, e.g., 20kHz to 70 kHz. In the preferred embodiment, the frequency during startupis set to 50 kHz. It can than be caused to sweep up at a particular rateuntil a change in impedance, indicating the approach to resonance, isdetected. Then the sweep rate can be reduced so that the system does notovershoot the resonance frequency, e.g., 55 kHz. The sweep rate can beachieved by having the frequency change in increments, e.g., 50 cycles.If a slower rate is desired, the program can decrease the increment,e.g., to 25 cycles which both can be based adaptively on the measuredtransducer impedance magnitude and phase. Of course, a faster rate canbe achieved by increasing the size of the increment. Further, the rateof sweep can be changed by changing the rate at which the frequencyincrement is updated.

If it is known that there is a undesired resonant mode, e.g., at say 51kHz, the program can cause the frequency to sweep down, e.g., from 60kHz, to find resonance. Also, the system can sweep up from 50 kHz andhop over 51 kHz where the undesired resonance is located. In any event,the system has a great degree of flexibility

In operation, the user sets a particular power level to be used with thesurgical instrument. This is done with power level selection switch 16on the front panel of the console. The switch generates signals 150 thatare applied to the DSP 60. The DSP 60 then displays the selected powerlevel by sending a signal on line 152 (FIG. 3(b)) to the console frontpanel display 12.

To actually cause the surgical blade to vibrate, the user activates thefoot switch 40 or the hand piece switch 34. This activation puts asignal on line 154 in FIG. 3(a). This signal is effective to cause powerto be delivered from push-pull amplifier 78 to the transducer 36. Whenthe DSP or microprocessor 60 has achieved lock on the hand piecetransducer resonance frequency and power has been successfully appliedto the hand piece transducer, an audio drive signal is put on line 156.This causes an audio indication in the system to sound, whichcommunicates to the user that power is being delivered to the hand pieceand that the scalpel is active and operational.

FIGS. 5(a) and 5(b) are flowcharts illustrating an embodiment of theinvention. When a good transducer and blade are driven in free standingair, the impedance measured by the generator is independent of the drivecurrent which corresponds to the specific power level in use, and thePower-Level vs. Power Delivered curve follows a quadratic relationship.Accordingly, upon measuring the power delivered at a single referencelow power level setting, where no transverse vibrations can be triggereddue to the low power level/drive current, the expected power deliveredat other power levels may be extrapolated from the measured singlereference low power level. This mathematical relationship holds trueregardless of the type of blade being utilized with the hand piece. Forpurposes herein, the term power includes power and any of itscomponents, including voltage, impedance, and current. Power alsoentails any other component that varies as a function of power, such asthe temperature of the transducer.

Typical maximum power levels, which are used by the ultrasonic generatorto drive the hand piece/blade, are shown in FIG. 4. Hand piece and bladecombinations exhibiting transverse behavior at level 5 do not exhibittransverse mode behavior at power levels 1 or 2, and very rarelyexhibits transverse mode behavior at power level 3. A hand piece/bladecombination will occasionally exhibit transverse mode behavior at powerlevel 4. Transverse behavior is indicated when the power into a handpiece/blade combination for a given load (in this case, while the bladeis operating free standing) is higher than expected for the particularpower level being applied to the blade.

Each power level is associated with a specific current at which thegenerator drives the hand piece transducer, and regulates across loadchanges on the blade. If the transducer/blade does not exhibittransverse vibrations, an increase of the power level will not increasethe transducer/blade impedance which is measured by the generator. Inthis case, the expected increase in power delivered to thetransducer/blade follows a known relationship (i.e., the power deliveredas a function of the drive current). This relationship is true in caseswhere the level of transducer/blade impedance measured by the generatoris independent of the drive current associated with the power levelsetting of the generator (i.e., when transverse vibrations are notpresent). The mathematical relationship of the power delivered as afunction of drive current level and transducer/blade impedance is aquadratic relationship, as shown in FIG. 9. In the preferred embodiment,the quadratic relationship is:

P _(L)=(I _(L))² *Z,   Eq. 1

where P_(L) is the power delivered to the transducer/blade, I_(L) is thecurrent delivered to the transducer (preset for each of the five powerlevels), and Z is the real part of the impedance.

If the transducer/blade exhibit transverse vibrations, an increase inthe power level will increase the transducer/blade impedance which ismeasured by the generator. As a result, when transverse vibrationsexist, the actual power delivered to the transducer/blade will exceedthe expected power. Generally, transverse vibrations are rarely presentat low power levels. This permits the performance of a reference powerconsumption measurement at a low power level setting to establishpass/fail power consumption levels for a high power level setting. Thereference power level measurement is a necessary measurement, becausethe transducer/blade impedance measured by the generator is dependantupon the blade used. An accurate indicator of the presence of transversemode vibrations is when the power consumption is greater than theexpected threshold at high power levels (see FIG. 10).

Accordingly, under the control of the program stored in the DSP ormicroprocessor 60 shown in FIGS. 3(a) and 3(b), the method of theinvention is implemented by using the ultrasonic generator to excite thehand piece/blade while it is being held in mid air, as indicated in step500. While the blade is still being held in mid air, the power deliveredto the hand piece/blade is measured at power level 1, as indicated instep 510. Using the measured power delivered to the hand piece/blade atlevel 1, the expected power for power level 5 is calculated, and apass/fail threshold is set for power level 5 based on the expectedpower, as indicated in step 520. The pass/fail thresholds are set at afixed percent above the calculated expected powers which are measuredwhen transverse vibrations are not present in the hand piece/blade. Inthe preferred embodiment, the threshold is set at approximately 10%above the expected measured power.

The actual power deliver to the hand piece/blade at power level 5 ismeasured, as indicated in step 530. Next, the actual measured power iscompared to the pass/fail threshold for power level 5, as indicated instep 540. If the actual measured power is greater than the respectivepass/fail threshold (step 550), then operation of the generator isinhibited, a “Transverse Mode Vibrations Present in Hand Piece/Blade”error code is stored in the generator and a “Bad Hand Piece” message isdisplayed on the LCD of the console, as indicated in step 555. On theother hand, if the none of the actual measured powers are greater thanthe respective pass/fail threshold, then the hand piece/blade is goodsince it does not contain transverse mode vibrations, as indicated instep 560.

In order to obtain the impedance measurements and phase measurements,the DSP 60 and the other circuit elements of FIGS. 3(a) and 3(b) areused. In particular, push-pull amplifier 78 delivers the ultrasonicsignal to a power transformer 86, which in turn delivers the signal overa line 85 in cable 26 to the piezoelectric transducers 36 in the handpiece. The current in line 85 and the voltage on that line are detectedby current sense circuit 88 and voltage sense circuit 92. The currentand voltage sense signals are sent to average voltage circuit 122 andaverage current circuit 120, respectively, which take the average valuesof these signals. The average voltage is converted by analog-to-digitalconverter (ADC) 126 into a digital code that is input to DSP 60.Likewise, the current average signal is converted by analog-to-digitalconverter (ADC) 124 into a digital code that is input to DSP 60. In theDSP the ratio of voltage to current is calculated on an ongoing basis togive the present impedance values as the frequency is changed. Asignificant change in impedance occurs as resonance is approached.

The signals from current sense 88 and voltage sense 92 are also appliedto respective zero crossing detectors 100, 102. These produce a pulsewhenever the respective signals cross zero. The pulse from detector 100is applied to phase detection logic 104, which can include a counterthat is started by that signal. The pulse from detector 102 is likewiseapplied to logic circuit 104 and can be used to stop the counter. As aresult, the count which is reached by the counter is a digital code online 104, which represents the difference in phase between the currentand voltage. The size of this phase difference is also an indication ofresonance. These signals can be used as part of a phase lock loop thatcause the generator frequency to lock onto resonance, e.g., by comparingthe phase delta to a phase set point in the DSP in order to generate afrequency signal to a direct digital synthesis (DDS) circuit 128 thatdrives the push-pull amplifier 78.

Further, the impedance and phase values can be used as indicated abovein a diagnositic phase of operation to detect if the blade is loose. Insuch a case the DSP does not seek to establish phase lock at resonance,but rather drives the hand piece at particular frequencies and measuresthe impedance and phase to determine if the blade is tight.

FIGS. 6(a) and 6(b) are flow charts illustrating another embodiment ofthe invention. When a good transducer and blade are driven in midair orin tissue, the Frequency vs. Power-Delivered Curve follows a knownrelationship. Accordingly, upon measuring the impedance of the handpiece at three nearby frequencies (i.e., the primary resonance andslightly off-resonance to above and below the primary resonance), theexpected impedance of the hand piece at other frequencies may beextrapolated from these prior measurements. Generally, such a relativerelationship exists for a variety of blades and shears.

One benefit of utilizing such a frequency-shift to detect transverseresonance is the test can be performed while the blade is in tissue. Theshift away from the primary resonance is very brief. As a result, thetissue load does not appreciably change during the momentary shift.Thus, the effects of tissue loading do not substantially influence theresonance frequencies and resulting capability to detect transversemodes. As a result, the diagnostic procedure can be performed while thesystem is in use, particularly at level 5 or level 4 where problems withheat generated by transverse modes are most likely to occur. The methodof the invention can be performed periodically, such as every 10seconds, while the generator is driving the hand piece/bade, during auser-initiated diagnostic and/or at other desired times, such as whilethe blade is being held in mid air or is in contact with tissue.

Periodically during use, such as during use at level 5 and/or level 4which are more likely to stimulate and evoke a substantial transversemotion than during use at level 1, level 2 or level 3, the drivefrequency is moved slightly away from the primary resonance for a briefperiod of time. In the preferred embodiment, the primary resonance ischanged by approximately 50 to 500 hertz and the brief period of time isapproximately 10 msec to 0.5 seconds. If the output power, outputcurrent or other parameter substantially shifts (i.e., the impedanceincreases appreciably, for example), then a transverse frequency isbeing driven or is close to being driven via stimulation from theprimary frequency. For example, the generator can be driving the bladeat the primary intended resonance and if a transverse mode resonance isclose by, it is highly likely to be more stimulated by the slight shiftin frequency towards that transverse mode resonance. Even a slight shiftfrom the primary resonance causes considerable energy coupling to anyclose transverse mode resonance. The difference in impedance and/orpower change when shifting from primary resonance to the slightly offresonance frequency is measured and compare to an expected impedancechange for a properly operating non-transverse condition system. If atransverse mode resonance is or is about to become problematic, theslightly off resonance impedance changes will be different for a goodvs. a problematic transverse mode hand piece. In the preferredembodiment, the changes measured are power level, current level,impedance, phase, or the like.

The comparison to a properly operating non-transverse mode conditionsystem may be based on established pass/fail criteria and/or byperforming a measurement when first using a particular blade to obtain abaseline measurement which is reliable. For example, the nearesttransverse frequencies are determined by sweeping the frequency of thedrive signal in the general vicinity of the expected primary resonanceand identifying any resonances which exist. Such a sweep is performedwhen a blade is first installed on the hand piece. Thereafter, thegenerator periodically drives the hand piece/blade at these particulartransverse frequencies to monitor any shift which occurs. Alternatively,if the type of blade attached to the hand piece is known, theestablished pass/fail thresholds for that blade can be more specificallydefined without the need to sweep the actual blade prior to use.

Under control of the program stored in the DSP or microprocessor 60shown in FIGS. 3(a) and 3(b), the method of the invention is implementedby using the ultrasonic driver unit to drive the hand piece/blade at aprimary resonance frequency, as indicated in step 600. In the preferredembodiment, the primary resonance frequency is 55 Khz.

The power delivered to the hand piece is measured, as indicated in step610. The primary drive frequency is shifted downward by approximately200 Hz, as indicated in step 620. A measurement of the hand pieceimpedance is performed, as indicated in step 630. A check is performedto determine whether the hand piece impedance at the primary resonanceis no more than 20% lower than the hand piece impedance measured at 200Hz below the primary frequency, as indicated in step 640.

If the hand piece impedance at the primary resonance is more than 20%lower than the impedance measured at 200 Hz below the primary frequency,then an alert or alarm is generated to indicate that a transversemalfunction is present, as indicated in step 650. If the hand pieceimpedance at the primary resonance is no more than 20% lower than thehand piece impedance at 200 Hz below the primary frequency, the drivefrequency is shifted upward by approximately 200 Hz above the primaryresonance frequency, as indicated in step 660. The hand piece impedanceis measured, as indicated in step 670. A check is performed to determinewhether the hand piece impedance at the primary resonance is no morethan 20% lower than the hand piece impedance measured at 200 Hz abovethe primary frequency, as indicated in step 680.

If the measured hand piece impedance at the primary resonance is no morethan 20% lower than hand piece impedance measured at 200 Hz above theprimary frequency, then a pause is initiated (step 685), and a return tostep 600 occurs. In the preferred embodiment, the pause is forapproximately 10 seconds. If, on the other hand, the hand pieceimpedance at the primary resonance is more than 20% lower than the handpiece impedance measured at 200 Hz above the primary frequency, then areturn to step 650 occurs, where the alert or alarm is generated toindicate that a transverse malfunction is present within the handpiece/blade. Alternatively, the power delivered to the hand piece may bemeasured, using a constant drive. When transverse mode vibrations arepresent, the power delivered to the hand piece will be appreciablygreater.

FIGS. 7 is a flow chart illustrating an alternative embodiment of theinvention. When a good transducer and blade are driven while being heldin free standing air, the Power-Delivered vs. Drive Current Level curveand Hand Piece/Blade Impedance vs. Drive Current Level curve follow aknown mathematical relationship. In the case of the Hand Piece/BladeImpedance vs. Drive Current curve, the mathematical relationship is anapproximately straight line (i.e., the impedance measured by thegenerator is independent of the drive current level), and in the case ofthe Power-Delivered vs. Drive Current Level curve the mathematicalrelationship is quadratic, as shown in FIG. 9. These mathematicalrelationships hold true irrespective of the type of blade being utilizedwith the hand piece.

In the presence of transverse behavior, the Power-Delivered vs. drivecurrent level curve and the Hand Piece/Blade Impedance vs. Drive CurrentLevel curve fail to follow the known mathematical relationships.Transverse behavior is indicated when the power into the transducer andblade combination for a given load (in this case, while the blade isoperating free standing or in mid air) is higher than expected for theparticular power level which is being applied to the blade (i.e. higherimpedance and higher power), as shown in FIG. 10.

Accordingly, under control of the program stored in the DSP ormicroprocessor 60 shown in FIGS. 3(a) and 3(b), while the blade is heldfree standing in mid air, the method of the invention is implemented byusing the ultrasonic driver unit to sweep the drive current levelapplied to the hand piece/blade from the minimum drive current level tothe maximum drive current level, as indicated in step 700. In thepreferred embodiment, the minimum current level is 100 mA RMS and themaximum current level is 425 mA RMS.

During the current sweep, the transducer voltage level and current drivelevel are monitored and stored in non-volatile memory located in thegenerator, as indicated in step 710. Using the stored voltage andcurrent data, the power delivered to the transducer is calculated, andthe Power-Delivered vs. Drive Current Level and Hand Piece/BladeImpedance vs. Drive Current Level response curves are generated, asindicated in step 720.

Using the generated response curves, an extrapolation is performed todetermine whether the Hand Piece/Blade exhibits transverse modevibrations which may create heat, as indicated in step 730. Theextrapolation comprises checking the generated response curves todetermine whether they represent a relationship which is equivalent to astraight line (e.g., for impedance comparisons) or a relationship whichis quadratic (e.g., for power comparisons). In the preferred embodiment,the quadratic relationship is in accordance with Eq. 1. If transversebehavior is present (step 740), i.e., the curves fail to follow theexpected mathematical 10 relationships (i.e., the relationships areexceeded), operation of the generator is inhibited, a “Transverse ModeVibrations Present in Hand Piece/Blade” error code is stored in thegenerator, and a “Bad Hand Piece” message is displayed on the LCD of theconsole, as indicated in step 745. On the other hand, if the curves areconsistent with the expected mathematical relationships, then the handpiece/blade is good since it does not contain transverse modevibrations, as indicated in step 750.

FIGS. 8(a) and 8(b) are flowcharts illustrating a preferred embodimentof the invention. Under the control of the program stored in the DSP ormicroprocessor 60 shown in FIGS. 3(a) and 3(b), the method of theinvention is implemented by using the ultrasonic generator to excite thehand piece/blade while it is being held in mid air, as indicated in step800. While the blade is still being held in mid air, the impedance ofthe hand piece/blade is measured at power level 1, as indicated in step810. Using the measured impedance of the hand piece/blade at level 1,the expected impedance for power level 5 is calculated, and a pass/failthreshold is set for power level 5 based on the expected impedance, asindicated in step 820. The pass/fail thresholds are set at a fixedpercent above the calculated expected power which is measured whentransverse vibrations are not present in the hand piece/blade. In thepreferred embodiment, the threshold is set at approximately 10% abovethe expected measured impedance.

The actual impedance of the hand piece/blade at power level 5 ismeasured, as indicated in step 830. Next, the actual measured impedanceis compared to the pass/fail threshold for power level 5, as indicatedin step 840. If the actual measured impedance is greater than therespective pass/fail threshold (step 850), then operation of thegenerator is inhibited, a “Transverse Mode Vibrations Present in HandPiece/Blade” error code is stored in the generator and a “Bad HandPiece” message is displayed on the LCD of the console, as indicated instep 855. On the other hand, if the none of the actual measuredimpedances are greater than the respective pass/fail threshold, then thehand piece/blade is good since it does not contain transverse modevibrations, as indicated in step 860.

In a further embodiment, whether the circuit is being driving in atransverse state, or whether transverse modes are about to occur isdetermined. Specifically, periodically during use of the system,preferably when operating at level 5, which is more apt to stimulatesubstantial transverse motion, the drive frequency is briefly changedsuch that it is slightly off resonance (i.e., above/below theprimary/main resonance). Here, the blade will still be substantiallydriven at its main resonance frequency. If a transverse mode vibrationis nearby, it is highly likely to be more stimulated by the slight shiftin drive frequency. The intent is to not extend too far from the primaryresonance peak (approximately 100 Hz to 500 Hz, for example) such thatthere is still a considerable amount of energy coupling to the primaryresonance and also to any nearby transverse mode vibrations.

Next, a check of the difference in impedance and/or power when movingfrom the primary resonance frequency to the slightly off resonancefrequency is performed. For example, a comparison of a measured powerchange to an expected power change for a properly operatingnon-transverse non-problematic system is performed. If a transverse modeis, or is about to become, problematic, the slightly off resonancechanges will be different for a good vs. a problematic hand piece/blade.The slightly off resonance check is brief and relatively transparent tothe user. In the preferred embodiment, the comparison of on resonancemeasurements and slightly off resonance measurements is performedbetween the power, the impedance, the phase shift, or another variable.The comparison is between at least one of an absolute value, a delta, arate of change and/or a 2^(nd) derivative.

If transverse mode vibrations are detected, the transverse activity canbe attenuated by shifting the primary resonance drive frequency suchthat it is shifted slightly off center of the resonance frequency. Inthe preferred embodiment, the frequency is shifted in a directionopposite to transverse sensitivity. The check for transverse activity isrepeated, and if the attenuations fail to halt the transverse behavior,operation of the ultrasonic system is terminated and/or the user usalerted to the undesirable condition. The present embodiment provides ameans to safely avoid overheating the blade, damaging the blade, oroverheating the hand piece.

In an alternative embodiment, a Multiple Level Drive Power vs. PowerDelivered relationship and/or a Multiple Level Drive Power vs. Impedancerelationship is used to detect or predict potential transverse modeproblems, along with an “over-drive” of the hand piece at one or severalpower drive levels beyond the normal range of power levels used. These“over-drive” power levels are particularly effective at rapidlyidentifying problematic or potentially problematic transverse modeconditions.

In another embodiment, the power delivered to the hand piece is measuredat multiple frequencies while a high power drive signal is applied tothe hand piece. Alternatively, an “over-drive” is used. Here, threefrequencies, i.e., a first frequency, a second frequency and a thirdfrequency, are measured in close proximity to each other. The firstfrequency is the primary resonance frequency, otherwise referred to asthe main or intended resonance frequency of the hand piece/blade. Thesecond frequency is slightly below the first frequency. The thirdfrequency is slightly above the first frequency.

Generally, power level 5 is the maximum power which is output duringuse. This power is the largest intended level for performing ameasurement of power delivered to the hand piece. However, in accordancewith the present embodiment, a power (i.e., a current) beyond Level 5 isbriefly applied (100 msec, for example) to the hand piece/blade. Thisbriefly applied power has a minimal impact upon tissue, but inputssubstantially more power into the hand piece to more effectively engageneighboring transverse resonant frequencies and more effectivelyidentify potential transverse mode problems. As a result, the“overdrive” provides a greater ability to identify potentiallyproblematic transverse modes, since this intentional excessive drive hasa greater ability to rapidly evoke a transverse resonance response thatwould not otherwise be seen, or be adequately prominent, if driving atthe normal maximum range associated with power level 5. Alternatively,multiple “over-drives” can be utilized to quickly identifynon-linearities between power levels, and thereby reveal potentialtransverse mode conditions and reduce the need to perform measurementsat lower operating levels.

Alternatively, the hand piece/blade is momentarily driven at one“overdrive” power level which is approximately 10% above the maximum ofpower level 5 when in use. If transverse modes are present or imminent,such an “overdrive” quickly reveals a dramatically higher power ordifferent impedance than normally expected at level 5. In an embodiment,comparisons are based on tables of commonly expected pass/fail limitswhich are indexed to blade types, such as automatic blade identificationor a manually entered blade type.

In another embodiment of the invention, the presence of transverse modevibrations is determined by monitoring both the transducer drive voltageand the transducer drive current to detect whether the Hand Piece/BladeImpedance vs. Frequency curve or the Power-Delivered vs. Frequency curvedeviate from the expected curve which is relatively steep. In accordancewith the present embodiment, while the blade is being held in midair orin tissue, the frequency is swept from a minimum frequency to a maximumfrequency. During the frequency sweep, the transducer voltage level andcurrent drive level are monitored and stored in memory located in thegenerator or in the blade. Using the stored voltage and current data,the power delivered to the hand piece is calculated, and thePower-Delivered vs. Frequency and the Hand piece/blade Impedance vs.Frequency response curves are generated. Using the generated responsecurves, an extrapolation is performed to determine whether the Handpiece/blade exhibits transverse mode vibrations or is in a marginalstate and therefore about to exhibit transverse mode behavior. Suchtransverse modes can create heat at undesired locations in the systemwhich can be hazardous. If a transverse mode resonance is determined toexist, an alarm or alert is generated by the generator to permit theuser to halt system use or take other appropriate steps. In alternativeembodiments, the generator automatically stops driving the hand piece.

In another embodiment, the presence of transverse mode vibration isdetected by calculating ratios of measured impedances at multiple powerlevels and determining the presence of transverse vibrations based onthe calculated ratios. In preferred embodiments, the impedance at level5 is measured and the impedance at level 1 is measured. Next, the ratioof the measured power levels is compared to a predetermined threshold todetermine whether transverse vibrations are present. In preferredembodiments, the predetermined threshold is 1.6.

In a further embodiment of the invention, predictive measures are usedto avoid or mitigate the onset of transverse resonances. Often,transverse resonances are stimulated by frequencies near or at theprimary resonance. The frequency “gap” between such resonances canshrink during use due to blade heating, which can eventually result ininadvertently driving the hand piece/blade at the transverse frequency.By periodically monitoring the nearest transverse resonance frequency,the gap between such resonant frequencies can be determined and therelative potential for inadvertently driving the hand piece/blade attransverse frequencies can be measured and used to predict potentialtransverse mode problems. For example, higher drive levels tend to heatup the blade faster, and hence more quickly reduce the gap. Thispredictive information is used to then halt driving of the transducer,alert a user that a potential transverse problem may be about to occuror alter the primary drive frequency to bias it slightly away from theideal resonance at a frequency further away from the transversefrequency. The size of the gap, the rate of change of the gap and/or the2^(nd) derivative of the gap can be used to provide predictiveinformation about the relative potential and/or existence of transversemodes.

In preferred embodiments of the invention, the current (i.e., “powerlevel”) is related to power by the relationship P=I²* Z, where thecurrent I is rms amperes, and Z is the real part of the impedance.Hence, at a first low level P₁=* Z₁. At a second high level P₂=I₂ ²* Z₂.In cases where Z does not increase (or decrease), then P₂/P₁=(I₂/I₁)².Accordingly, in the case where actual measurements of P₁ and P₂ revealthat P₂/P₁>(I₂/I₁)², then transverse vibrations are present in the handpiece.

Using the method of the present invention, an indication of whether ahand piece which failed the power level tests will exhibit transversevibrational modes is provided. Moreover, as a consequence of themathematical relationship holding true irrespective of the type ofblade, the method can be used with any hand piece/blade combination. Inaddition, the method also ensures safe operation of the hand piece bypreventing overheating of the blade, thus avoiding damage to the bladeor injury to an individual using the hand piece.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method for detecting transverse vibrations inan ultrasonic hand piece, comprising the steps of: applying a drivesignal to an ultrasonic hand piece using an ultrasonic generator;measuring an electrical characteristic delivered to the hand piece/bladeat a first electrical characteristic level while the hand piece/blade isheld in midair; calculating an expected electrical characteristic usingthe first electrical characteristic level; setting a pass/fail thresholdlevel for an actual measured electrical characteristic based on theexpected electrical characteristic; measuring the actual electricalcharacteristic delivered to the hand piece/blade; comparing the measuredactual electrical characteristic to the pass/fail threshold; if themeasured actual electrical characteristic is less than the pass/failthreshold, displaying a first message on a liquid crystal display of thegenerator; and if the measured actual electrical characteristic isgreater than the pass/fail threshold, displaying a second message on aliquid crystal display of the generator.
 2. The method of claim 1,wherein the first message is a “Hand Piece/Blade Passed” message.
 3. Themethod of claim 1, wherein the first electrical characteristic level isa power level.
 4. The method of claim 3, wherein said calculating stepcomprises the step of: calculating the expected electricalcharacteristic for a level five setting.
 5. The method of claim 4,wherein the electrical characteristic is power.
 6. The method of claim3, wherein the level 5 setting is approximately 425 ma RMS.
 7. Themethod of claim 3, wherein the power level is approximately 100 ma RMS.8. The method of claim 1, wherein said step of displaying a secondmessage comprises the step of: storing a “Transverse Mode VibrationsPresent in Hand Piece/Blade” error code in the generator; and displayinga “Bad Hand Piece” message on the liquid crystal display of thegenerator.
 9. The method of claim 1, wherein the drive signal has afrequency of approximately 20 Khz to 70 KHz.
 10. The method of claim 1,wherein the pass/fail threshold is approximately 10 percent of theexpected electrical characteristic.
 11. The method of claim 10, whereinthe electrical characteristic is power.
 12. A method for detectingtransverse vibrations in an ultrasonic hand piece, comprising the stepsof: sweeping a drive signal having a voltage and current applied to anultrasonic hand piece/blade using an ultrasonic generator; monitoringand storing parameters applied to the hand piece/blade in non-volatilememory located in the ultrasonic generator; calculating an electricalcharacteristic delivered to the hand piece based on the stored voltagesand stored currents; generating response curves based on the storedparameters; performing an extrapolation using the response curve todetermine whether the hand piece/blade exhibits transverse modes; if thehand piece/blade exhibits transverse modes, displaying a message on aliquid crystal display of the generator.
 13. The method of claim 12,wherein the message is a “Hand Piece/Blade Contains Transverse ModeVibrations” message.
 14. The method of claims 12, wherein said applyingstep comprises the step of: applying the drive signal from a minimumdrive current level to a maximum drive current level.
 15. The method ofclaim 14, wherein the minimum drive current level is approximately 100ma RMS and the maximum drive current level is approximately 425 ma RMS.16. The method of claim 12, wherein the response curves are at least oneof a Power-Delivered vs. Drive Current Level curve and a HandPiece/Blade impedance vs. Drive Current level curve.
 17. The method ofclaim 12, wherein the extrapolation comprises the step of: checking thegenerated response curves to determine whether any curve represents atleast one of a straight line and a relationship which is quadratic. 18.The method of claim 12, wherein the electrical characteristic is power.19. The method of claim 12, wherein the parameters are voltages andcurrents.